The field of the present invention relates to data readers and in particular to light generation and collection systems suitable for reading of symbols such as barcode labels.
A barcode label comprises a series of parallel dark bars of varying widths with intervening light spaces, also of varying widths. The information encoded in the barcode is represented by the specific sequence of bar and space widths, the precise nature of this representation depending on which particular barcode symbology is in use. Typical methods for reading barcodes comprise generation of an electronic signal wherein a signal voltage alternates between two preset voltage levels, one representing a dark bar and the other representing a light space. The temporal widths of these alternating pulses of high and low voltage levels correspond to the spatial widths of the bars and spaces. It is this temporal sequence of alternating voltage pulses of varying widths which is presented to an electronic decoding apparatus for decoding.
One common type of bar code readers are spot scanners in which a source of illumination is moved (i.e., scanned) across the barcode while a photodetector monitors the reflected or backscattered light. For example, the photodetector may generate a high voltage when a large amount of light scattered from the barcode impinges on the detector, as from a light space, and likewise may produce a low voltage when a small amount of light scattered from the barcode impinges on the photodetector, as from a dark bar. The illumination source in spot scanners is a typically a laser, but may comprise a coherent light source (such as a laser or laser diode) or non-coherent light source (such as light emitting diode). A laser illumination source may offer advantages of higher intensity illumination which may allow barcodes to be read over a larger range of distances from the barcode scanner (large depth of field) and under a wider range of background illumination conditions.
The reading spot of the scanner may be manually moved across the bar code, this type of reader being typically referred to as a wand. Alternately,the spot may be automatically moved or scanned across the bar code in a controlled pattern. A scanning mechanism may comprise a rotating mirror facet wheel, an oscillating mirror, or other suitable means for repetitively moving the illumination beam. The path followed by the scanned illumination beam is referred to as a scan line. Typically, an individual scan line extends across the barcode for the barcode to be successfully read unless specialized piecing software (known as stitching) or electronics are utilized. In addition to the scan engine, a barcode scanner may also employ a set of scan pattern generating optics to produce a multiplicity of scan lines in various directions from the scanner and at varying orientations, thereby allowing barcodes to be read over a large angular field of view and over a wide range of orientations (i.e., a multi-dimensional scan pattern). The scan pattern generating optics typically comprise a set of mirrors aligned at varying angles, each of which intercepts the illumination beam during a portion of its motion and projects it into the region in front of the barcode scanner, hereinafter referred to as the scan volume. Each mirror or mirror set, in conjunction with the scanning mechanism, produces a scan line at a particular position and at a particular orientation.
Another type of data reader is an image reader, such as a CCD reader (charge coupled device), in which an entire line of the bar code image is focused onto a detector array. A CCD reader typically includes a light source to illuminate the bar code to provide the required signal response. For the purposes of this description, the word "scanner" may refer to data readers of both the spot scanner type and the line scanner imaging type. The following description will focus on barcode reading, but is generally applicable other types of symbol reading or object identification.
FIG. 1 illustrates a typical laser barcode scan module 100 in which an illumination beam 102 from a laser 104 is directed by a steering mirror 106 through a lens 108. Lens 108 may serve as a collimation or focusing lens for the illumination beam 102 in addition to its primary function of collecting light 109 from the barcode 110 and focusing it onto photodetector 112. After passing through the lens 108, the illumination beam 102 impinges on a scan engine 114, which may comprise a rotating mirror facet wheel, an oscillating mirror, holographic disk or other scanning mechanism for scanning the illumination beam 102 across the barcode 110. The reflected or refracted light 109 from barcode 110 is collected by lens 108 and, bypassing steering mirror 106, is focused onto detector 112, where the light intensity is converted to an electrical signal. The electrical signal is then passed from the barcode scan module 100 to signal processing and/or decoding electronics 116.
Though the optical layout of barcode scan module 100 was useful, the present inventors have recognized that there are still improvements that can be effected. The optical layout occupies a relatively large amount of space due to the need for steering mirror 106 to direct the illumination beam 102 along the optical collection path. The steering mirror 106 also decreases the collection efficiency of lens 108 by obstructing a portion of the collected backscattered light 109. This inefficiency in turn necessitates the use of a larger diameter, longer focal length collection lens 108, and/or the use of a higher power laser 104. Further, since the focusing requirements for the scanned illumination beam 102 and the collection system are typically quite different, additional illumination beam focusing optics may be required between laser 104 and steering mirror 106. Alternatively, lens 108 may comprise a complex multi-focus optic, with a central portion 108a configured for the scanned illumination beam 102 and the outer portion configured for collection.
The layout of module 100 is also sensitive to relative misalignment of laser 104, steering mirror 106, lens 108, and detector 112. Detector 112 and lens 108 define a volume of space in front of the barcode scan module from which reflected light 109 is most efficiently collected and focused onto photodetector 112. Unless laser 104 and steering mirror 106 are aligned so that the illumination beam 102 traverses this collection volume, the barcode scan module will not function efficiently. The scan module 100 may therefore be complex and expensive to manufacture, requiring a large number of parts and tight manufacturing tolerances. It is also bulky due to the constraints imposed by the relatively large number of components comprising the optical system.